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366                         METALLURGY OP IRON AND STEEL.
Mckel steel has the same modulus of elasticity as carbon steel; it has greater resistance to shock and torsional strains and to compression. This is not due to hardness, as it is readily cut by ordinary tools, and soft steel cannot be made hard merely by the addition of nickel.
Mckel steel has superior stiffness, but bends to greater angles before rupture; plates of this metal are not weakened by punching as much as those of carbon steel. In bridge construction the usual allowance for expansion can be made. The shearing strength is greater than with carbon steel. Mckel segregates only slightly even in the largest ingots.
There are other elements used to make special alloys with iron, some of these metals being of considerable importance. Tungsten and chromium are both employed to give tool steels extreme hardness, their characteristic being that no quenching or tempering is required. These alloys, however, do not come under the head of structural material, and will therefore not be considered here.
SEC. XVIIj.—Influence of oxide of iron.—The last step in the making of a heat of steel is the addition of the recarburizer to wash the oxygen from the bath, but this action is not perfect, and the ex-actrelation is notgenerallyunderstood. The amount of oxygen taken from the metal will be measured in a rough way by the amount of manganese and other metalloids that are burned during the reaction. This is particularly true of acid practice. In basic work there is oftentimes a very considerable loss of manganese through the presence of free oxygen in the slag. This occurs in the acid furnace, but less frequently. The loss of manganese in recar-burization is a function of the quantity which is added. In other words, a 'reduction in the percentage of manganese added to an open-hearth bath at the time of tapping means a reduction in the amount of manganese oxidized, and this proves that the reaction is not perfect, and that an increasing amount of oxygen must remain in the metal as the content of manganese decreases; but a reasonable proportion of this oxygen can hardly exert any marked deleterious influence, else the fact would long ago have been known in. some more definite form than the suppositions and theories which are occasionally founded on exceptional phenomena. Assuming that high oxygen will more likely be found in steels low in manganese/it may reasonably be expected that any bad effect'will be